The present invention concerns a process for recovering electrolytic copper with a high degree of purity by means of reduction electrolysis -- hereinafter to be called electrowinning--from copper sulphate solutions obtained in the hydrometallurgical working up, such as, for example, a sulphur leaching, of materials containing copper.
As, generally speaking, the quality of electrolytic copper cannot clearly be designated by its analytic composition alone on account of the reciprocal action of the different impurities, the physical properties of this copper, i.e. its electric conductivity and its semi-hard temperature, also serve as a yardstick for its degree of purity.
Hence, the present invention concerns a process for recovering electrolytic copper with an electric conductivity of at least 58.8 Siemens (mho) x m/mm.sup.2 (101.4% IACS, respectively) and a maximum semi-hard temperature of 200.degree. C by means of electrowinning from copper sulphate solutions obtained in the hydrometallurgical working up of materials containing copper.
The copper sulphate solutions, obtained in the hydrometallurgical working up of materials containing copper, usually contain as impurities both such constituents as interfere with electrolysis or have an unfavourable influence upon the purity of the cathode copper owing to simultaneous separation or the formation of electrolytic inclusions and such constituents -- hereinafter to be called neutral salts --as show neutral behaviour towards the electrolytic process. The process in accordance with the present invention consist of these copper sulphate solutions being subjected to preliminary treatment--here inafter to be called purification of the solution --prior to being fed into electrowinning for the purpose of removing such elements, contained in the solution, as interfere with the electrolytic process and the quality of the electrolytic copper. In accordance with the present invention, this removal of the disturbing elements is effected in two steps.
The cupiferous materials, conventionally used for the production of the copper sulphate solutions, are the following:
A. CUPRIFEROUS SULPHIDE CONCENTRATES OR ORES WHICH, FOR THE PURPOSE OF PRODUCING THE COPPER SULPHATE SOLUTION, ARE SUBJECTED TO SULPHATIZING ROASTING, FOR EXAMPLE, WITH AQUEOUS SULPHURIC ACID LEACHING BEING EFFECTED SUBSEQUENTLY, OR TO OXIDIZING PRESSURE LEACHING IN A SULPHURIC ACID MEDIUM;
B. CUPRIFEROUS OXIDE CONCENTRATES SUCH AS, FOR EXAMPLE, CARBONATES, OXIDES, AND SILICATES, WHICH, FOR THE PURPOSE OF PRODUCING THE COPPER SULPHATE SOLUTION, ARE LEACHED WITH AN AQUEOUS SULPHURIC ACID SOLUTION; AND
C. INTERMEDIATE METALLURGICAL PRODUCTS AND RECYCLING MATERIALS SUCH AS, FOR EXAMPLE, COPPER MATTE, COPPER DROSS, COPPER PRECIPITATES FROM CHEMICAL PROCESSES FOR WORKING UP IN THE WET WAY, FOR EXAMPLE, CUPROUS OXIDE, BASIC COPPER SULPHATES, COPPER-ARSENIC CEMENTATES, AND OTHER INTERMEDIATE METALLURGICAL PRODUCTS WHICH, IF NECESSARY, ARE SUBJECTED TO OXIDIZING LEACHING OR TO LEACHING WITH SULPHURIC ACID ONLY.
Owing to the composition of the cupriferous starting materials and depending upon the extraction process, with a usually given level of concentration, the copper sulphate solutions, obtained in the different leaching processes, generally contain so many disturbing secondary constituents that there is an unfavourable influence on either the electrolytic process or the quality of the electrolytic copper produced, or both, if these disturbing elements are not largely removed by means of purification of the solution prior to the copper sulphate solution being fed into electrowinning.
The disturbent factors in question are known to be the following:
a. the iron which has an unfavourable influence upon electolytic performance by decreasing current efficiency and considerably increases corrosion of the cathode suspension loops at the surface of the bath; b. the elements Se, Te, Bi, Sb, As, Sn, Pb, and S, which, depending upon their concentration and, to a certain extent, also on the manner of their inclusion, deteriorate the physical properties, i.e. conductivity and semi-hard temperature, of the electrolytic copper;
c. the chloride ion, which, if present in too high a concentration, can interfere with the electrolytic process in several ways, that is to say, by rapid corrosion of the leadantimony anodes, by pitting corrosion of the high-grade steel used for the production of starter sheet, and, finally, with even higher concentrations present, by incorporating chloride ions in the copper precipitated;
d. the solids content in the electrolyte, which, depending upon the type of solids, can cause nodule formation.
Other secondary constituents such as, for example, salts of Ni, Co, Zn, Mg, Al, Mn, etc., are known to not interfere with electrolysis, provided that the concentration of these so-called neutral salts is kept below a certain upper limit which, by the way, depends upon the electrolytic conditions, i.e. upon the copper concentration, the current density, the electrolytic temperature, etc. As a rule, the current density is adjusted to the composition of the electrolyte for the purpose of precipitating the copper in the form of sufficiently fine crystals, so that electrolytic inclusions, particularly those of sulphur and oxygen, but also those of secondary constituents, possible present, such as, for example, the above-mentioned salts of Ni, Co, Zn, Mg, Al, Mn, etc., are kept at a minimum. The concentration of the neutral salts is maintained below a certain upper limit by means of a continuous or discontinuous discharge of an adequate volume of the electrolyte, as is conventional practice.
Thus, the present invention concerns a process for producing a pure copper sulphate solution from any copper sulphate leach which may contain one or several of the above-mentioned disturbing impurities and neutral salts, with the whole of the copper sulphate leach being subjected to a twostep purification of the leach during which the concentration of the disturbing impurities is reduced so much that there are obtained by subsequent electrowinning copper cathodes of highpurity quality in accordance with the standards stated at the beginning.
As is well known from experience, it is impossible for copper cathodes of the high-purity quality required to be recovered by means of electrowinning from copper sulphate leaches without purifying the whole of the copper sulphate leach for the purpose of removing, the disturbing elements or, at least, reducing their concentration below a limit below which the influence becomes neglible. Even if, in practice, the concentration of the disturbing elements, present in the copper sulphate leach, is maintained at a constant level by either discharging part of the copper sulphate solution or removing the disturbing elements from part of the copper sulphate solution, such measures are not sufficient at all to produce copper cathodes with the high degree of purity desired.
That is to be illustrated by examples from practice:
a. Sulphide copper concentrates, containing 45% Cu and 2.5% Co, are subjected to sulphatizing roasting, with the roasted material being leached with an aqueous sulphur acid solution for the purpose of extracting the copper (see P. H. Charles and P. Hannaert, "Copper Metallurgy". Edited by R. P. Ehrlich. AIME. New York, 1970 pp. 240 - 259). The copper sulphate leach obtained is subjected for the purpose of removing the selenium to a single-step purification of the leach subsequent to the leaching residue being separated.
The leach having been clarified without filtration, the copper sulphate solution is fed into the electrowinning cycle. There are removed only disturbing impurities such as Fe, As, etc. and neutral salts such as, among others, Mg, Al, and Co, by means of discharging part of the electrolyte volume from the electrolyte cycle. Electrolysis is performed from an electrolyte, containing 30 g/l Cu, 40 g/l Co, 70 g/l H.sub.2 SO.sub.4, and 2.5 g/l Fe, the current efficiency amounting to 85 - 90%; the cathodes, thus obtained, contain 99.7% Cu. These cathodes do not come up to the high degree of purity of the cathodes which can be produced by the process in accordance with the present invention; therefore, they have to be subjected to further refining, that is to say, to fire refining. Hence, it follows that the single leach purification step, effected for the purpose of removing the selenium, is not sufficient to produce cathodes with a high degree of purity. Moreover, in view of a current efficiency of 85 - 90%, the electrolytic performance is rather middling.
b. According to another example from practice (see C. B. Kettering and K. L. Power, AIME Annual Meeting. New York. Feb. 16 - 20, 1958. Preprint 5817 P 10), copper ores, containing about .86% copper in all -- 0.46% oxide copper and 0.40% sulphide copper --, are leached in vats with an aqueous sulphuric acid solution containing iron. The copper sulphate solution, containing about 25 g/l Cu, 6.5 g/l Fe in all, 0.7 g/l of which is Fe.sup.3.sup.+, is fed into electrowinning and electrolized with a current density of about 150 A/m.sup.2 and at a temperature of 40.degree. C. The electrolyte contains 20 g/l Cu, 17 g/l H.sub.2 SO.sub.4, 6.5 g/l Fe -- 3.7 g/l of which Fe.sup.3.sup.+ is. The current efficiency is very low, e.g. 76%, the cathodes contain about 99.0% Cu. The cathodes produced are not equal to the high degree of purity achieved by the process according to the present invention.
c. According to a third example (see L. R. Verney, J. E. Harper, P. N. Vernon in "Electrometallurgy". Ed. Henrie, Baker, Cleveland. 1969. pp. 288 - 291). a cppper sulphate solution is obtained by means of aqueous sulphuric acid leaching of sulphide and oxide copper concentrates subjected to sulphatizing roasting. There is no purification of the leach; there is only a partial removal in a side-stream (by-pass) of the iron in solution. Prior to being fed into electrowinning, the copper sulphate leach contains the following impurities;
______________________________________ g/l g/l ______________________________________ Fe, in all 10 Mg 10 Fe.sup.3.sup.+ 6.0 Al 10 Co 3.0 m/gl Ni .6 Bi 40 Mn 2.0 Se .7 ______________________________________
Electrolyis is effected at about 185 A/m.sup.2, current efficiency amount to only about 77%. These cathodes are not equal to the high degree of purity desired, on account of the high amounts of Se, Bi, and Pb as shown in the following table:
______________________________________ ppm ppm ______________________________________ Fe 2 Se 5 Ni 1 Pb 15 Bi 3 As .5 ______________________________________
d. According to a last example from practice (see Transactions AIME, Vol. 106. pp. 559 - 608), anoxide copper ore, containing about 1.7% Cu and 0.2 - 0.3% Cl.sup.-, is leached in a leaching basin with an aqueous sulphuric acid solution. The copper sulphate leach, containing considerable amounts of impurities, had the following average composition:
______________________________________ g/l g/l ______________________________________ Cu 40 Mg 8 H.sub.2 SO.sub.4 10 Mn 8 Fe 15 Cl 1.5 Al 15 SiO.sub.2 3 ______________________________________
Subsequent to being dechlorinated with cement copper to 0.05 g/l Cl.sup.-, the copper sulphate solution with a solids content of about 100 - 200 mg/l is decopperized step by step to about 10 g/l Cu in the electrolytic way, the average current density amounting to 130 A/,.sup.2, and the temperature being 40.degree. C. The current efficiency is very low, amounting to an average 50%. On account of their considerable impurity content, the cathodes, containing 99.7% Cu, are subjected to fire refining.
It follows from all the example given, that it is impossible to obtain electrolytic copper with a high degree of purity by means of electrowinning from a conventional copper sulphate leach containing impurities.
There is a process, it is true, known to be capable of obtaining a pure copper sulphate solution from a conventional copper sulphate leach containing impurities, that is to say, a process by which the Cu.sup.2.sup.+ ion is at least partially removed from the copper sulphate leach, containing impurities, by means of extraction with the aid of a liquid ion exchanger and conveyed into a sulphuric solution. While this process is suitable for working up a copper sulphate leach with a low concentration such as, for example, 2-10 g/l Cu (see Mining Magazine. September, 1974. pp. 165-169; World MIning, April, 1971, pp. 46-48), its application will be the more unattractive for metallurgical and economical reasons, the higher the copper concentration in the copper sulphate leach. This is due to the fact that, in the presence of higher concentrations, there has to be used an extraction solvent which combines with the copper more firmly; hence, there must be present more highly concentrated sulphuric solutions if the Cu.sup.2.sup.+ ions are to be conveyed into the sulphur acid. Therefore, electrowinning must be effected with a higher H.sub.2 SO.sub.4 concentration of, for example, &gt;200 g/1 H.sub.2 SO.sub.4, which is known to cause considerable operating trouble such as, for example, corrosion of the conventional Pb-Sb anodes and conveyance of Pb and Sb into the copper cathodes. Other types of anodes, such as, for example, the expensive D.S.A. anodes -- i.e. Dimensional Stable Anodes; Ti coated with mixed oxides of Pt metals -- or Pb-Ca anodes should be mentioned in this context.
Also, there is this known alternative to the above-mentioned extraction with the aid of an extraction solvent fixing the copper more firmly: the process in question is carried out with a conventional extraction solvent used in the working up of a low-concentration copper sulphate leach, with the sulphuric acid, formed by means of an intermediate neutralization with a calcium hydroxide suspension, being fixed as gypsum and the gypsum being separated from the three-phase system (see DBP 24 10 158). However, this has the disadvantage of requiring working with a three-phase system.